Saturation Superheat Subcooling

refregeration cycle.pdf

To understand superheat or subcooling we must first understand Saturation

#1 Pressure (PSIG) converted to a Saturated temperature.
Suction saturated is the Blue, low pressure, suction gauge pressure converted to a  vapor/ Dewpoint/ suction saturated temperature.
Liquid Saturated is the Red, high pressure, head pressure, liquid gauge converted to a liquid/ Bubblepoint/ head saturated temperature.
We convert the pressure to the saturated temperature of the refrigerant with a Pressure Temperature  (TP) chart  (aka saturated PT chart), an app or an automatic conversion wit digital guages.

#2 Liquid and Vapor both exist together.
In a tank of refrigerant, as long as there is any amount of liquid and vapor, it will follow the  saturated TP chart.
In the evaporator,  immediately after the metering devices unstill all the liquid is gone it is a saturated mixture. Ideally this will be most of the evaporator coil before all the liquid boils away.
In the condenser, the middle of the condenser starting approximately 2 rows from the top  to approximately 2 rows form the bottom the refrigerant will be saturated, liquid vapor mixture. 

#3 Change of state, Latent heat
Most heat is absorbed or rejected  as the refrigerant changes state from a liquid to a vapor or from a vapor to a liquid, without changing temperature (latent heat). A massive amount of heat is absorbed or rejected as it changes state but the temperature stays the same as long as there is liquid and vapor. this is also referred to as hidden heat because the temperature does not increase even though  its absorbing or rejecting heat.
In the evaporator, the refrigerant changes state  from a liquid to a vapor absorbing a massive amount of heat (BTU) from the air even thought the temperatures is the same. This is often referred to as evaporator temperature aka evaporator saturated temperature. It will be this temperature until all of the liquid has changed state to a vapor.
In the condenser, where the refrigerant is changing state from a vapor into a liquid. This is where the refrigerant is rejecting the most amount of heat back into the air outside. It will be this temperature until all the vapor  has changed state into a liquid.

#4 Condensing, Boiling/Evaporating
The condenser is where the refrigerant changes from a vapor to a liquid, rejecting a massive amount of heat into the cooler outdoor air. The condensing  pressure and saturated temperature will be  hotter than the outdoor air. As the refrigerant gives up heat to the cooler air, more of the refrigerant changes state to a liquid at the same temperature (latent). Condending temperature and liquid saturated temperature can be used interchangeably in this application. 
The evaporator is where the refrigerant is violently boiling as the refrigerant changes state from a liquid to a vapor. the low pressure and Heat from the air makes the refrigerant boil, as the refrigerant boils it absorbs heat from the air. As the air gives up its heat to the refrigerant,  more of the refrigerant changes state to a vapor.
Boiling is a cooling effect.
In our lives we have associated boiling to be hot because water boiling on a stove is hot to our bodies. However according to the saturated temperature of water at 0 PSIG it remains at a cool 212F untill all the water is gone. nomatter how much heat you apply to the water it stays at 212. If you where cooking roman, it would stay at a cool 212 until all the water was gone, the the noodles would increase in heat until it burned. By droping the pressure of water we can drop the boiling temperature and make water boil at room temperature. Every substance has a differentl boiling temperatures. Oxygen at 0 PSIG boils at -297F (saturated temp) and nitrogen boils at -320F (saturated temp). If you where to dip your hand in either of those boiling substances your hand would be frozen as the heat leaves your hand and moves into boiling nitrogen or oxygen. Boiling is a cooling effect.
R 410A boils at -60F at 0PSIG. if you where to tunr the tank upside down , the refrigerant would be boiling , changing state at -60F absorbing heat from antying warmer than it. If this was your hand you would have sever frostbite. If the evaporator pressure was at 119PSIG the saturated/Boiling temperature would be 40F. The refrigerant under that pressure is boiling changing state at 40F, absorbing heat from the cooler 75F air traveling on the outside of the evaporator coil the Warmer air makes the refrigerant boil and change state. the refrigerant boiling absorbs heat out of the air, cooling the air.
The refrigerant is boiling in the evaporator but since the term boiler was already used in plumbing the term Evaporator was used. Evaporation is also the change of state from liquid to vapor but generally in as a slower pace. This sometimes cause's confusion but we can use Evaporator temperature, Boiling Temperature, Suction Saturated temperature interchangeably. 


Superheat is the amount of heat added to a VAPOR above its saturation temperature.

It ensures we are providing only vapor to the compressor while still cooling it and a way to measure how much refrigerant is in the evaporator.

In the evaporator the refrigerant changes state from a liquid to a vapor, after all the liquid is gone, the refrigerant is still a lower temperature than the air so there will still be heat transfer. As the heat transfers from the warmer air to the cooler refrigerant vapor, the vapor will start to sensibly increase in temperature.
Sensible heat, a change in temperature without a change in state. There is very little heat transfer in sensible heat vs Latent

Oxygen has a saturation (boiling) temperature of -297F at 0 PSIG. If I was to pour oxygen out and the temperature was 297 or below, the oxygen would be a liquid. If I was to pour the liquid oxygen out at a temperature of -296F the liquid would boil to a vapor.
If the temperature was 0F and I was to pour the liquid oxygen out, it would boil to a vapor and that vapor would sensibly increase in temperatures superheat, increasing in superheat.
Saturated temperature of oxygen  -297F
-Actual temperature of of the area 0F
oxygen vapor would be superheated 297F above its saturation temperature. If you were outside 0F would be "cold" but to the oxygen it woul be highly superheated.
The room I am in right now measured with a thermometer is 75F, and oxygen has a saturated temperature of -297F at 0psig
Saturated temperature -297F
- actual measured temp 75F
= Superheated vapor of 372F, meaning the oxygen I am breathing is superheated 372 above its saturation temp of -297.
We are breathing in superheated vapor!!!! (even if its “cold” to us)

Evaporator Superheated Vapor
We can record the  actual temperature  of the suction line leaving the evaporator with a thermometer  ______________F
We convert the suction pressure to a boiling/saturated temperature (Dew for blends)   _____________________________F
The actual evaporator suction line temperature - the suction saturated temperature equals the  evaporator Superheat________F

Actual suction line temperature ______F
- Suction saturated (dew) ____________F
= Evap Superheat  Vapor ____________F


Low Superheat (Flooded Evaporator)
If the superheat showed 0 then we  are still at saturation. That means we have a flooded evaporator and will have liquid and vapor back to the compressor which cannot handle any liquid. If we have 0 superheated vapor we are at saturation meaning liquid and vapor. We don't know how much liquid and vapor but any liquid to the compressor is bad. Generally we want no less than 5F of superheat in any condition.
Low superheat washes the oil away from the compressor bearings quickly reducing the life and in some cases enters the compression cylinder causing immediate slugging and broken parts.
Low superheat is often from Low airflow, low air temperature, fouled (dirty) coils, an over feeding or oversized metering device and an overcharged system.

 High Superheat (starved Evaporator)
If we have too much superheated vapor (high superheat)  then we likely don't have enough refrigerant in the evaporator.  Without enough refrigerant in the evaporator, the liquid boils  to a vapor quickly. The remaining vapor has more time, surface area and distance to travel through the evaporator increasing the amount of sensible superheated vapor as it does.
If the superheated vapor is too high, there is not enough saturated refrigerant entering the evaporator, not enough refrigerant changing state, and not enough refrigerant absorbing heat meaning the capacity of the system and the cooling effect will be much lower.
high superheat also leads to compressor overheating damaging the compressor.
High superheat is from undercharged system, refrigerant restriction, clogged or underfeeding metering device, or high load ( hot startup, pulling in attic air)

What should the Superheat be? This will depend on the metering device!

With a Thermostatic expansion valve (TXV TEV) or an Electronic expansion valve (EXV EEV)
If we have the proper charge, the proper airflow on a clean coil these metering devices will try to maintain superheat for us. In refrigeration they are often adjustable, but in residential ac they are often non adjustable. Generally a TXV for air conditioning will try to maintain an evaporator superheat of 8-12F. In refrigeration this can be much lower.
Because these metering devices open and close to control the superheat they system works the most efficiently by maintaining the best amount of refrigerant in the evaporator coil but more importantly it helps protect the compressor from refrigerant floodback or refrigerant overheat when the conditions change. A TXV has a bad reputation for lack of understanding, misdiagnosing and poor installation practices. See TXV for more information

Fixed orifice metering device, Capillary tubes or fixed hole/piston metering devices.
These metering devices cannot adjust to changing conditions.
When the outdoor temperature rises, the head pressure also rises. We now have more pressure pushing against the liquid refrigerant and pushing more refrigerant through the fixed hole into the evaporator. We will need the temperature of the air entering the condensing coil.
The indoor air temperature and indoor humidity affects how fast the refrigerant in the evaporator coil boils to a vapor. By collecting the return air wet bulb temperature, it will include the effects of sensible air temperature and latent humidity temperature of the air affecting the refrigerant in the evaporator coil.

By taking the outdoor air entering condensing coil and the indoor return air wet bulb temperature we can find the target superheat for the indoor coil. We can enter those numbers into a formula (     ) a chart ( see below), a calculator (ac service tech) or an app ( hvac school app/Measurequick) to find the target superheat, or what the superheat should be.  One day it could be as low as 0 flooding the compressor and another day it could be as high as 35 overheating the compressor so it's less than ideal but many techs like it because it does not break, ignoring the damage it can cause to the compressor.

 We also have total superheat meaning superheat  taken at the suction line at the outdoor unit or before entering the compressor.

Actual suction line temperature ________F
- Suction saturated (dew)   _____________F
= total Superheated vapor  ______________F

The total system's superheat will usually be higher than the evaporator superheat because the temperature of the vapor is still increasing and absorbing heat as it travels in the suction line. Several factors affect how much superheat is gained.
The length of the suction line.
The location of the suction line.
The thickness of insulation on the suction line
How well sealed the suction line insulation is.
Check with the specific compressor to find the operating limits. Some compressors want a minimum of 20F superheat while another compressor of the same brand wants a maximum of 20F superheat.


Discharge superheat.
The compressor sucks in a low temperature low pressure superheated vapor, and pumps out a high temperature high pressure superheated vapor.  As the superheat coming in rises the discharge line superheat coming out of the compressor will also rise. Once the  discharge temperature 6" from the compressor rises above 225 F we have loss of lubricity and oil breakdown from compressor overheat that is one of the leading causes of compressor failure.  If the superheat entering the compressor is good, but the superheat leaving the compressor is high, that can be a  sign of poor oil return to the compressor. it's a good habit to check discharge line temperature and  discharge superheat. 

Discharge line temperature _______________F
-Liquid saturated temperature (dew) ______F
= Discharge superheat ____________________F

After the refrigerant leaves the compressor and enters the condensing coil it must be de-superheated first. Once we sensibly cool the refrigerant vapor back to saturation, then condensing can take place. This usually takes only 1 or 2 rows for this to happen.


Subcooling is a sensible measure of heat removed from a liquid below saturation

Subcooling is a sensible measurement so very little heat is removed but it does provide us with an important measurement.

The metering device can only meter liquid refrigerant, so it's essential to have liquid feed to the metering device. If we have 0 of subcooled liquid, then we are at saturation meaning liquid and vapor entering the metering device.

With a Fixed orifice metering device we don't focus on subcooling but it is an important number to know we are providing liquid to the fixed metering device. As the temperature outside goes up the pressure goes up so more liquid will be pushed out of the condenser into the evaporator. As the temperature outside increases, subcooling will be lower.
As the temperature outside drops the pressure drops and more subcooled liquid will remain or stack in the condensing coil 

Subcooling will be very important with a Thermostatic expansion valve TXV TEV and also Electronic Expansion Valve (EXV EEV)
Often the tag will have the label “indoor TXV subcooling ___”. This means if there is a TXV located inside the target subcooling (what the subcooling should be) is “___”. The manufacturer designs the condensing coils to hold a specified amount of liquid refrigerant in the condensing coil. The last row or 2 of the condensing coil typically is set aside for subcooling. I have seen manufactures state a subcooling as low as 5 and as high as 25, so it's important to check with that specific model of condensing unit.  If you are in a bug rush and the tag is gone and you cannot find the PDF installation manual for that unit, 10 subcooling will often get you by. (Excluding refrigeration equipment)

Condenser Liquid saturated temperature (bubble) ___________F

-The thermometer measured  actual liquid line temp _________F
=Subcooled Liquid…………………………………………………………__________F

Flooded condenser means the subcooling is higher than your target.
You have too much refrigerant in the condensing unit.
Could be overcharged, a restriction in the liquidline, Low ambient conditions and more.

Starved condenser means subcooling is lower than the target.
You don't have enough refrigerant in the condensing unit

Could be undercharged, oversized metering device, malfunctioning metering device, sensing bulb not attached properly and more

It's important to note you may have the proper subcooling at the condensing unit and not have subcooling at the metering device. Ideally we would have a service port before the TXV to check this or at least a sight glass to see there were no vapor bubbles. If the liquid line runs into an attic or hot mechanical room, the liquid refrigerant can heat back up, raising the actual refrigerant temperature, reducing subcooling as the refrigerant temperature gets closer and closer to saturation. Many times it helps insulate the liquid line but a restriction can cause a pressure drop with the same results. 

Refrigeration equipment usually will not have a subcooling if there is a liquid receiver. The liquid receiver makes a liquid seal providing a full column of liquid to the metering device. Since the receiver is liquid and vapor in 1 container, you could be at 1 or even 0 subcooling but still be completely in liquid form. A sight glass will help you see into the system to ensure there are no vapor bubbles.

Don't add refrigerant bases on sight glasses alone. During a hot startup, the Txv could let all the refrigerant into the evaporator coil, so its best to let the system pull the box temperature down first. It is possible to check the subcooling leaving the condenser, before the liquid receiver.

Some refrigeration systems have a special subcooler added to increase the subcooling before the metering device. 

Copy of Target Superheat new1.xlsx - Sheet1(1).pdf